The Atomic Theory

Atomic Theories

The idea of an atom -- the smallest particle of matter -- has intrigued mankind since the beginning of civilization.  Throughout the centuries the "view" of the atom has changed.  New ideas, and new technologies have influenced the model of the  atom.  This view of the atom is still a Theory and therefore it is still subject to change.  The  modern model of the atom is called the Quantum Model. The chart below summarizes the various atomic models that have been developed during the course of history.

Scientist & approximate Date		Name of Model, Sketch and main idea of theory		Importance and Improvement on previous model	Shortcomings - Problems or why was it changed
Democritus   c.300 BC		Atom the indivisible particle Atomos (in ancient Greek) means "that which cannot be further broken down into smaller pieces".		Talks about the atom as the smallest particle of matter. Defines the atom as an indivisible particle Explains certain natural occurrences such as the existence of elements	Does not give a scientific view of the atom only a conceptual definition Does not talk about subatomic particles (Electrons, Protons, Neutrons)
Dalton   c.1800		The solid sphere model Atoms are seen as solid, indestructible spheres (like billiard balls)		Explains a lot of chemical properties such as how atoms combine to form molecules Explains chemical change better than the  Particle Theory Confirms the basic Laws of Chemistry: Conservation of Mass & definite Proportions	Does not include the existence of the nucleus Does not explain the existence of ions or isotopes Does not talk about subatomic particles (Electrons, Protons, Neutrons)
J.J. Thomson  c.1850		The raisin bun Model or the chocolate chip cookie model :  Atoms are solid spheres made-up of a solid positive mass (or core) with tiny negative particles embedded in the positive core.		Infers on the existence of electrons and protons Introduces the concept of the nucleus Infers on the relative nuclear density and atom mass of different atoms	Does not explain the existence of electrons outside the nucleus does not explain the role of electrons in bonding Does not talk about neutrons therefore can't explain radioactivity and the existence of isotopes
Rutherford c. 1905		The Planetary Model Famous Gold Leaf Experiment proves that the nucleus is positive and the electrons are outside the nucleus.		First real modern view of the atom Explains why the electron spins around the nucleus (Bohr's Contribution) Proposes that the atom is really mostly empty space	Does not place electrons in definite energy levels around the nucleus Doesn't include neutrons in the nucleus Does Not relate the valence electrons atomic charge
(Neils Bohr) Bohr- Rutherford c. 1920		Electrons in Definite energy Levels around  the nucleus Used atomic spectra to prove that electrons are placed in definite orbitals (called shells) around the nucleus.		Explains the role of valence electrons in bonding Relegates the number of valence electrons to the Periods of a periodic table Fully explains ionic and covalent bonding Places electrons in definite energy levels 2 e- in the first 8 e- in the second 8 e- in the third	It does not explain the shapes of molecules or other abnormalities that result form unevenly shared pairs of electrons (such as the abnormal behaviour of water, the difference in Carbon-Carbon Bonds between diamond and graphite etc..)
Modern Theory Many Scientists Contributed.  Some of the more famous  are: Schroedinger Einstein Luis De Broglie Max Planck Frank Hertz Maxwell Fermi		Quantum Mechanical Model or Electron Cloud Model The analogy here is that of a "beehive" where the bees are the electrons moving around the nucleus in a "cloud" of energy levels.			Advanced Theories will explain bonding and other facts about the behaviour of atoms and their chemical and physical properties in forming new compounds.

Other important facts about the particles of an atom:

Subatomic Particle	Symbol	Charge	Relative Mass*	Location
Proton	p+	positive	2000	nucleus
Electron	e-	negative	1	orbits around nucleus
Neutron	n0	neutral (zero)	2000	nucleus

*Relative mass means that is the electron has a mass of 1 unit, the proton and neutron will have a mass 2000 times that of the electron.

The mass number (also known as atomic mass or atomic weight) and the atomic number from the Periodic Table are very important numbers because they tell us how many subatomic particles are contained in a given atom.

The atomic number tells us the number of electrons and the number of protons., i.e. Atomic Number = Number of electrons = Number of Protons.  The atomic mass tells the total number of particles in the nucleus, i.e. Atomic Mass = # of protons + number of neutrons.